How does Torque, power, and specific fuel consumption change with AFR? And why is it so?

I found graphs relating these in books as well as websites but I couldn't find any good explanations.I have attached an Image I found on the internet.

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2 Answers 2


The graph tells you how it is, i'll tell you why it is so.

The reason why a leaner AFR (than stoich) is more efficient, is that it burns hotter.(but slower) It is so in every combustion. Compare it to a campfire, it will burn hotter when you start blowing in it. There's an excess of air, so the fuel won't have any problem combusting, if that excess is not too big.

A hotter combustion has two upsides:

  • The higher temperature will cause a more complete combustion, so less fuel is spent unburned or incompletely burned. That means more energy is extracted out of the fuel.
  • The higher temperature also means more pressure is made in the cylinder, which makes the piston do more work. Which again, puts more energy of the fuel to good use.

But why is a richer ratio better for more torque? (and thus more power)

That's because rich AFR's burn faster.(until a certain richness) That means pressure is made quicker after TDC. And so more torque is made. And torque times rpm is power. With leaner ratio's that pressure occurs later, which doesn't enable the engine to make as much power. Rich ratio's also mean more fuel can be used with the same air. That means more heat can be generated. NB: heat is different from temperature.

Also, because richer mixtures burn cooler, they're kinder on the engine and so enable it to make more power without things getting destroyed or melted.

The downside is that it isn't proportional, so you need proportionally more fuel to get more power. That's why leaner ratio's are more efficient. Specific fuel consumption says something about the power made per fuel flowrate. That's essentially the same as how much energy is extracted from a certain quantity of fuel.

There's more to it, but this covers the basics and should answer all your questions. If you leave a comment about it, I can send you a PDF that explains all there is to know about the combustion process in engines if you want. Dont forget to mark the answer to your question ;)


That graph tells you all of it:

  • highest NOx at best economy :)

  • highest CO at best power,

CO just about lowest at target A/F range...

Some references that you may find useful are:

  1. Judge, High Speed Internal Combustion Engines https://archive.org/details/cu31924003632191

  2. https://www.amazon.com/High-Speed-Internal-Combustion-Engine-Ricardo-F-R-S/dp/0216890268

Both will help "round out" what you need to know.

Also see the diagram in the answer here:


  • But why is that ratio(slightly leaner than stoichiometric) gives the best economy? And why is the other ratio gives the highest power? Higher the fuel burnt,higher the power.The highest amount of fuel that can be completely burnt is at the stoichiometric ratio.So why doesnt it give higher power?
    – Mohan
    Commented Aug 23, 2018 at 16:23
  • Is it because of the damage to the engine (due to high temperatures) at the stoichiometric ratio(or slightly leaner ) we dont use it?Is that why the richer ratio is cnsidered to give the highest power?
    – Mohan
    Commented Aug 23, 2018 at 16:32
  • So check out : en.wikipedia.org/wiki/Lean-burn#Chrysler_Electronic_Lean-Burn
    – Solar Mike
    Commented Aug 23, 2018 at 16:36
  • There are books by Cummins, Ricardo has several, Judge has a few that really explain what is going on with the basic principles and also explain the theory - all well worth reading...
    – Solar Mike
    Commented Aug 23, 2018 at 16:39

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